Railway signal control systems typically use the track circuit block as the basic element of train location, and communication and control. Electrical signals applied to the length of track comprising a block is shunted by the rail vehicle axle and the change in signal is detected and is used to indicate a track block that this occupied. In addition, such track circuits also can be used to detect for broken rail, and establish communication from wayside equipment to moving rail vehicles, including, for example cab signals. Because of the operating requirements associated with track block signals the equipment used in each track circuit must provide for a vital operation of that track circuit. While block signals give reliable indication of the vehicle position, the limiting factor is the length of a given block. When a vehicle crosses two adjacent 1,000 foot block sections, the signal apparatus will detect the vehicle within a 2,000 foot length of track. Because train operation depends upon the conditions in front of and behind moving vehicles such 2,000 fool vehicle indication may effect operation in over a mile of track. When it is desirable to operate a high frequency of trains (short headways) such as in rush hour mass transit systems, the safe headway between trains must be maintained at a minimum distance so as to permit a high operating frequency of service. One of the ways this can be achieved is by increasing the number of individual track circuits and decreasing the length of each track circuit. However, to obtain shortened track blocks requires a proportionally higher number of track circuit equipment and can become cost prohibitive. Since many trains are operated either automatically or manually based upon the train conditions received through cab signal equipment, the train information available to cab signal is uniform within the block and cannot take into account information or conditions such as grade that may exist within a portion of a block. Track conditions which are appropriate for the train at the entering section of a block may be non-optimum for uphill sections in the exiting end of a block. Presently the information would default to the reduced condition which would be unnecessary in uphill grade areas. Such default does not result in optimum train operation. This disadvantage can be overcome by using a larger number of discrete track circuits. If the track circuits comprise 100 foot blocks, the headways can be significantly increased over that of the 1,000 foot blocks. However, unfortunately such 100 foot track circuits would require ten times the track bonds, vital track interlocking, and vital logic. It is therefore desirable to obtain the effects of a large number of small interval track circuits without the cost of installing and maintaining such large number of track circuits. In typical track circuit systems the vehicle speed is controlled via speed data transmitted to each vehicle as a function of track circuit occupancy. The vital wayside distributed logic generates what applicable speed data should be transmitted to the vehicles by monitoring the states of all the track circuits in a particular control line. Thus as a vehicle occupies a particular track circuit, the vital wayside logic determines what speed data to send to the vehicle via cab signal as a function of how many track circuits are unoccupied and other train conditions.
Other rail vehicle signal systems do not use traditional track circuits but instead use a moving block system. The moving block system uses an automated train control system in which a following train receive information of the velocity and position of a train ahead of it. A central control function has a continuous dialogue with all trains on the system. The central control knows the velocity and position of each train on the system at all times. A vital train to wayside communication system provides position information to each train concerning the respective lead train to it. In some systems the central control function also provides velocity information concerning the lead train to the respective following train. On-board calculations then compute the speed profile to maintain at least a safe braking distance between itself and the lead train. The moving block system uses vital logic at the central control facility to provide the position of each train on the system, and to determine which information is fed to each train. Advantages of such a system are the lack of equipment associated with discrete track circuits, and the moving block system can result in reduced headways since the train control is based upon .safe braking distances to the specific location of the lead train rather than assuming the lead train to be occupying a whole track circuit block. Some of the disadvantages of such a moving block system are the reliance upon a central control facility to vitally process the information and transmit that vital information across the system. Failure at the central control facility can result in a system-wide shut-down as no information will be available to any train on the system.